PPPoE has two differents stages, first one is PPP Discovery stage that contains four steps when a host discovers the MAC address of peer (Concentrator) and the PPPoE session ID.
In the fact, the Mac address and PPPoE_SESSION_ID uniquely define a ession.
The relationship between the peers is a simple client/server when a client asks server(Concentrator) for informations to establish
the session.

The Frame sent is a simple Ethernet frame where the ETHER_TYPE is set to either 0x8863 (Discovery Stage) or 0x8864 (PPP Session Stage).

3. Client to server: Request (PPPoE Active Discovery Request)
PADR:
* Host receive one or more PADO packet and has to choice one
* Choice is based on AC-Name or Services offred
* Host send one PADR packet to Concentrator
* Destination is the unicat Ethernet address of Cencentrator
* code field is set to 0x19 and session id is set to 0x0000

5. Either end to other end: Termination (PADT)
* packet sent bu host or Access Concentrator
* session is established
* Destination address is unicast
* session is the SESSION_ID generated
* code field is set to 0xa7